Towards a better understanding of protein affinity for polystyrene nanoplastics: Investigation of surface charge effects, interaction mechanisms and aggregation kinetics.

This study investigates the mechanisms governing nanoplastic-protein interactions, aggregation, and colloidal stability between bovine serum albumin (BSA) and polystyrene nanoplastics (PS NPls) with opposite surface charges under controlled conditions at pH 7.4. Positively charged amidine latex (180 ± 20 nm) and negatively charged sulfate latex (220 ± 20 nm) PS NPls were characterized over pH 3-10, then studied in ultrapure water (UPW) and 10 mM HEPES. When BSA concentration was varied at fixed PS NPls concentration (30 mgL-1), cationic PS NPls (+) rapidly adsorbed BSA, inducing charge neutralization and aggregation at low BSA (≈ 4 mg L-1) through reduced electrostatic repulsion and protein bridging. At higher BSA concentration (20 mg L-1), surface saturation led to protein corona formation, charge inversion, and colloidal restabilization. Anionic PS NPls (-) remained dispersed, with no detectable aggregation or charge inversion by DLS and ζ-potential. In a BSA-rich model system (50 mg L-1) with varying PS NPls concentrations, low cationic PS NPls concentrations (< 4 mg L-1) produced stable BSA aggregates, while higher concentrations (> 4 mg L-1) yielded well-dispersed corona-coated particles. These interaction states formed rapidly and remained stable over 48 h. PS NPls (-) showed no significant interaction or kinetic evolution. Similar trends in UPW and HEPES indicate that ionic screening modulated but did not alter the charge-dependent mechanisms. Together, these findings highlight the central role of surface charge in controlling PS NPls-protein interactions and provide a mechanistic basis for future studies in complex biological media.